On 2014-10-22 22:34, Benjamin Herrenschmidt wrote: > On Wed, 2014-10-22 at 16:17 +0200, Jan Kiszka wrote: >> I thought about this again, and I'm not sure anymore if we can use >> ACPI >> to "black-list" the incompatible virtio devices. Reason: hotplug. To >> my >> understanding, the ACPI DRHD tables won't change during runtime when a >> device shows up or disappears. We would have to isolate virtio devices >> from the rest of the system by using separate buses for it (and avoid >> listing those in any DRHD table) and enforce that they only get >> plugged >> into those buses. I suppose that is not desirable. >> >> Maybe it's better to fix virtio /wrt IOMMUs. > > I always go back to my initial proposal which is to define that current > virtio always bypass any iommu (which is what it does really) and have > it expose via a new capability if that isn't the case. That means fixing > that Xen thingy to allow qemu to know what to expose I assume but that > seems to be the less bad approach.
Just one thing to consider: feature negotiation happens after guest startup. If we run a virtio device under IOMMU control, what will we have to do when the guest says it does not support such devices? Simply reject operation? Jan -- Siemens AG, Corporate Technology, CT RTC ITP SES-DE Corporate Competence Center Embedded Linux
